Odds are good that you've done one of two possible things.
If you have been changing file associations, you might have changed the launch association with EXE files. I don't think you can do this using normal tools like the Explorer, but you can definitely do it using registry edits.
If you have installed "interesting" software either intentionally or as a victim of a "drive by download", odds are good that the software wasn't 64 bit aware. There is a relatively common registry hack used by many types of programs that "follow" EXE execution that will cause this behavior, although most of those seem to latch onto Media Player instead of NotePad.